1. Field of the Invention
The present invention relates to liquid ejection recording heads for ejecting liquid to a recording medium, and specifically to a liquid ejection recording head for ejecting a plurality of droplets of multiple volumes to a recording medium.
2. Description of the Related Art
The resolution offered by color inkjet printers using thermal inkjet technology is increasing rapidly. In particular, in recording heads for forming images, the resolution of nozzles from which droplets are ejected is increasing yearly, such as from 600 dpi to 1200 dpi.
As for the size of ink droplets for forming images, in particular, color ink droplets ejected from a recording head, the size is decreasing yearly from, for example, about 15 pl to 5 pl, then to 2 pl for reducing graininess in halftones in gray-scale images, and halftones and highlights in color photo images.
However, for printing rough images not requiring high resolution, such as color graphs in reports, recording heads for producing small droplets and printing high-resolution images cannot meet demands for high-speed printing, because of the large amounts of output data and time required for data transfer.
To accommodate high-speed printing, it is desirable that recording heads be capable of forming images with relatively large droplets and small amounts of data. For high-quality printing, on the other hand, it is desired that the size of droplets be adjusted to minimize the graininess of images. That is, it is required that a group of recording head nozzles for the same color can eject ink droplets of different sizes.
In response, Japanese Patent Laid-Open No. 08-183179 (corresponding to U.S. Pat. No. 6,309,051) discloses means for ejecting ink droplets of different sizes from the same nozzles. In this case, ink channels communicating with the same nozzles are provided with electrothermal transducers of different sizes. Bubbles created by these electrothermal transducers cause ink droplets of multiple sizes to be ejected from the same nozzles.
The specification of U.S. Pat. No. 6,137,502 discloses an inkjet print head having large and small nozzles arranged in a staggered manner, and through which large and small droplets are ejected.
However, in Japanese Patent Laid-Open No. 08-183179, since droplets of different sizes are ejected from the same ink channels, the speed of supplying ink from the rear of the nozzle varies depending on the size of the droplets. In this case, it is difficult for a serial-type recording apparatus to eject droplets of different sizes through one scan of the recording head. It is thus required to eject droplets of different sizes (such as large, medium, and small) through multiple scans of the recording head. That is, since droplets of different sizes cannot be ejected at the same frequency, it is difficult to adjust the size of droplets to accommodate the formation of high-resolution images.
As for the specification of U.S. Pat. No. 6,137,502, the inkjet print head is provided with the same number of large and small nozzles. If the amount of ink to be ejected is set to be large, image quality is degraded in high-quality gray-scale printing (photo printing) while there is no particular problem in high-speed printing, where a large amount of ink is ejected. On the other hand, if the amount of ink to be ejected is set to be small, an increase in the number of print passes causes speed degradation while photo image quality is improved.